This invention relates to a method of applying a coating onto a substrate, and a corresponding apparatus. More particularly, this invention relates to utilizing first and second rapid and selective heating zones to efficiently provide a high gloss coating on continuously moving sheet, strip or blank material.
Liquid roller coating lines are known in the art, and may apply solvent or water-based paints/coatings to metal strip through the use of roller-coating machines. Unfortunately, environmental regulations have made such coating lines undesirably expensive in view of the need for solvent containment and incineration systems. Additionally, there is a finite limit to the thickness of a coating that can be effectively applied using such systems.
Accordingly, powder coating of strip material has been developed in the industry. This normally involves applying electrostatically charged dry plastic powder to a strip, and then passing the strip with powder thereon through a convection oven where the powder is melted and cured through a cross-linking process. An example of a powder-coating system is disclosed in U.S. Pat. No. 5,439,704, the disclosure of which is hereby incorporated herein by reference. Reference is also made to FIG. 1 herein, taken from the ""704 patent.
As shown in FIG. 1 of the ""704 patent, the powder-coating system includes input region 1, powder-coating booth 3, heating chamber 5, quench 7, and output region 9. When metal strip 11 is being processed, it is suspended through booth 3 and oven 5 between a pair of entrance rolls 13 and catenary roll 15. After the powder-coated strip 11 exits booth 3, the strip enters oven 5. The thermoset powder material on strip 11 melts and cures into a coating. The curing phase involves cross-linking of molecular chains of the thermoset plastic to form the final hardened material. In one example discussed in the ""704 patent, a polyester hybrid powder coated strip is held within oven 5 for approximately 25-30 seconds at a temperature of 475xc2x0 F.
Unfortunately, conventional heating processes have been found to be undesirable for a number of reasons. Additionally, when gasses within the thermoset material are not permitted to exit prior to curing, the finished product may suffer from the xe2x80x9corange peel effectxe2x80x9d, thus having a mottled surface (i.e. bumpy surface). This may occur when the powder-coated metal strip is heated at too fast a rate to too high a temperature. It has also been found that convection ovens are not particularly well suited for precisely controlling thermoset-coated material temperatures. Convection ovens also suffer from excessive dirt problems.
In view of the above, it is apparent that there exists a need in the art for an improved method for coating continuously moving strip (e.g. coil steel, coil aluminum, fabric, blanks, etc.) with thermoset material. There also exists a need in the art for an improved method of heating and/or curing thermoset material, so as to result in a superior finished product. It is a purpose of this invention to fulfill any and/or all of the above-described needs in the art, as well as other needs which will become apparent to the skilled artisan from the following detailed description of this invention.
It is an object of this invention to provide an efficient method of coating continuously moving sheet, strip or blank material with thermoset material.
Another object of this invention is to utilize first and second adjacent rapid response ovens/furnaces in order to efficiently heat and cure thermoset material coated onto continuously moving material, and the ovens may preferably include induction ovens and Infrared ovens that have a rapid response permitting precise selection and control over heating of the thermoset material.
Another object of this invention is to provide an efficient method and apparatus for coating steel, aluminum, other types of metal, fabric, and the like with thermoset material to a desired thickness.
Another object of this invention is to heat thermoset powder material applied to a continuously moving substrate in a manner such that the resulting coated (e.g. painted) product has high gloss.
Another object of this invention is to provide a method of coating a moving substrate with thermoset powder, heating the coated substrate to a first temperature, and thereafter heating the coated substrate to a second higher temperature in order to obtain a superior final coated product.
Still another object of this invention is to fulfill any and/or all of the above-listed objects.
This invention further fulfills any or all of the above described needs and/or objects by providing a method of making a coated article comprising the steps of:
electrostatically applying a thermoset powder coating onto at least one major surface of a continuously moving substrate and thereby providing a thermoset coated substrate;
moving the thermoset coated substrate into a first induction oven and heating the substrate and thermoset powder coating thereon to a first temperature in the first induction oven sufficient to substantially melt the thermoset powder;
moving the thermoset coated substrate from the first induction oven into a second induction oven and heating the substrate and thermoset coating thereon to a second temperature in the second induction oven sufficient to effect substantial cross linking of the thermoset, wherein the second temperature is higher than the first temperature; and
moving the substrate with cured thermoset coating thereon from the second induction oven to a quenching area for quenching.
This invention will now be described with respect to certain embodiments thereof, along with reference to the accompanying illustrations.